1. Field of the Invention
The present invention relates to an arrester.
2. Description of the Related Art
In a conventional type of polymer arrester in which an internal element including a zinc oxide element is directly molded with silicon rubber, for example, a plurality of zinc oxide elements are provided in an FRP (Fiber Reinforced Plastics) pipe in a stacked manner, electrodes are provided on top and bottom of the zinc oxide elements, a pressing spring for supporting the zinc oxide elements is provided between one of the electrodes and the zinc oxide elements, and an outer circumferential surface of the FRP pipe is covered by an outer polymer coat. The conventional arrester configured in the above manner achieves a mechanical strength with an insulating support member such as FRP arranged around the zinc oxide elements. Generally, the FRP in which a glass fiber is extended in one direction has an excellent mechanical strength against a tensile load exerting on a glass fiber direction, but not against a load in a direction perpendicular to the glass fiber direction. For this reason, when an electrode is fixed with FRP in which a hole processed portion is provided, for example, if a bending load is applied to the arrester, the load is concentrated on the hole processed portion, possibly resulting in breakage beginning at the hole processed portion so that a high mechanical strength cannot be expected. Therefore, a method of fixing the FRP and the electrode and the glass fiber direction of the FRP are important factors to obtain a high mechanical strength in the polymer arrester.
In a conventional polymer arrester disclosed in Japanese Patent Application Laid-Open No. 2002-75709, zinc oxide elements are stacked between a pair of electrodes, and a plurality of transverse U-shaped FRP clamp members are provided at a regular interval in a circumferential direction to nip a large diameter portion of each of the electrodes. The FRP clamp member includes no processed portion described above, and an FRP spiral member for preventing the FRP clamp member from being disengaged is provided on an outer circumference of the FRP clamp member.
However, because the FRP spiral member is provided to bundle the FRP clamp members in the conventional arrester disclosed in Japanese Patent Application Laid-Open No. 2002-75709, there may be a gap between an inner surface of the FRP spiral member and the outer surfaces of the FRP clamp members, by which the FRP clamp members and the electrode are not solidly fixed. The rigidity against the bending load at this time is lower than a case where the FRP clamp members and the electrode are solidly fixed. In such a structure of the conventional arrester, measures to achieve an arrester having a sufficiently high mechanical strength may include increasing the number of the FRP clamp members, increasing a cross-sectional area of the FRP clamp member, or increasing the rigidity of the arrester by winding the FRP spiral member in a tighter manner. However, such measures not only cause a size increase of the arrester and a complicated structure but also lead to an increase in cost of the arrester.
Meanwhile, because the conventional arrester employs the FRP spiral member, the thickness of an outer coat of the arrester in a radial direction increases, causing a size increase of the arrester and a complicated structure as described above. At the same time, as a usage amount of the silicon rubber increases, the size and the cost of the arrester increase accordingly. In a molded-type polymer arrester, when a short-circuit current flows, the silicon rubber is cracked by an arc gas generated in the arrester so that the arc gas is discharged from a side surface of the arrester. However, if the thickness of the silicon rubber is increased in the radial direction, the silicon rubber becomes hardly cracked, possibly failing to meet a predetermined pressure discharge performance.
The present invention has been achieved in view of the above problems, and an object of the present invention is to obtain an arrester having a high mechanical strength without causing any size increase.